The research has established a case control study in which women seeking hospital care for spontaneous abortion (cases) were compared with women having term pregnancies in the same hospital (controls), matched to cases by age and payment status. Smoking, and a history of a previous spontaneous abortion is common among cases. The history of a prior induced abortion is not common among cases, nor is the intake of sacharin during pregnancy. The products of conception were systematically examined, and its chromosomal composition obtained. It was found that proportion of conceptions with normal chromosomes was higher among women who smoked during pregnancy, and among those with a history of previous spontaneous abortions, than it was for the series as a whole. These results held after maternal age had been taken into consideration. We intercept these findings as coherent with the case-control study in confirming a raised risk of abortion in these women, and also in establishing that the excess abortions are of normal chromosomal constitution. Also, conceptions of women with prior reduced abortion were similar to the series as a whole in respect of the proportion karyotypically abnormal, again confirming the case control finding that there was no excess in these women.